Hardfaced plate is a product manufactured to resist the abrasion of bulk materials such as earth, rock, ores, and other silicious materials. It is used in the construction and the protection of fabrications of many kinds.
It consists of a base plate of any suitable analysis which has been weld overlaid with an alloy which contains typically up to 40% chromium carbide.
Depending upon the alloy content and method of application, the overlay can contain several types of chromium carbides. Only the primary type of carbide has the high abrasion resistance needed to make this product outwear other overlays or hard metals of a low cost type. This is identified by the formula M.sub.7 C.sub.3 where M is an alloy of iron and chromium.
Analysis of the individual carbides cannot be done with any common commercial equipment, but the carbide can be identified by its form and shape in a micrograph of the deposit.
Common mild steel plate is used as the base plate for nearly all manufacture of this product as it is sufficient to supply whatever strength and shock resistance are needed and is most cost effective.
The preferred method of manufacture uses the bulkweld process wherein alloy powder and wire are melted together in a welding arc and simultaneously welded to the base plate while melting an amount of base plate to obtain a weld bond, such as set forth in U.S. Pat. No. 3,076,888. Other patents illustrating hardfacing are U.S. Pat. Nos. 3,000,094; 3,060,307; 3,062,948; 3,407,478; 3,494,749; 3,513,288; 3,517,156; 3,588,432; and 3,609,292.
As the overlay is made by welding, it is not capable of precise parameters. There will be a typical variance between melting of about 20% or less base metal on the one hand and a lack of bond (no melted base metal) on the other hand. With lack of bond the overlay can break off, and with the heavier melting of base metal there is dilution of the overlay material with iron with the result that the type of carbides formed are low in abrasion resistance. In prior art processes, over 15% melted base material is not acceptable.
In the present state of the art all of the iron intended to produce the alloy is supplied by the electrode, usually a wire but it can be a strip. Whatever amount of base metal is melted is for the purpose of producing a bond only.
In the present art the unintentional melting of base plate (beyond the small amount needed for bond) progressively reduces the formation of primary chromium carbides which are the principal constituents which give the product its high abrasion resistance.
In a typical operation the welding parameters are critical within a narrow range. Because of the nature of a welding process, the operation is difficult to control within that range. A relatively small heat input above the desired level will cause greater base metal melting and undesirable structure while a relatively small reduction in current will cause lack of bond allowing the overlay to break off.
In the present invention, the base metal is intentionally melted and intentionally forms part of the alloy surface being produced. This insures bonding of the alloy surface to the base, and the metal supplied from the metal base to be part of the alloy produced is of much lower cost than that supplied by the electrode wire.
In the manufacture of hardfaced or corrosion resistant plate the most efficient application and easiest to produce quality is obtained when employing the bulkweld system of welding as described above. More particularly, in this system the arc which does the melting and the iron required of the alloy facing are supplied by steel (iron) electrode (wire) continuously fed by automatic welding equipment. The carbide or corrosion resistant forming elements required and the other elements needed are supplied by powder metered into the weld zone in a precalculated ratio to the amount of wire used. As these elements fuse and mix in the weld puddle, they together produce the desired facing alloy.
In order to melt a greater amount of base metal, a larger current is needed than in current overlay welding. In utilizing a larger current in the current prior art processes, a greater amount of base metal is melted, but the greater current overloads the electrode wire (3/32") and causes it to melt very fast and thus still supply most of the iron needed in the weld.
It would be highly advantageous to increase the quality of the abrasion resistant surface to provide assurance of a weld bond, to increase the speed of application, and to decrease the cost of materials and labor.